Currently, generators connected to variable speed wind turbines are used in the production of electric power that reaches the end users through the three-phase electric power distribution grid. Such generators have been known for nearly a hundred years. See, for example, U.S. Pat. No. 5,083,039 issued Jan. 21, 1992 to Richardson et al. (hereinafter “the '039 patent”), which describes a system in which the generator stator is connected to the grid via an active rectifier, which is a first inverter, and a second inverter connected via a DC link. In this system, which is not double-fed, the generator torque is controlled by controlling the currents or voltages applied to the stator, and a power factor controller controls active switches in the second inverter to determine the current phase angle and magnitude and, thus, control the reactive power applied to the grid. However, this system has significant cost because the power converter has to handle the entire power of the generator and, therefore, must be very robust.
Because of the problems with the system of the '039 patent discussed above, recent wind turbine generators have tended to control the system through the rotor in a doubly-fed asynchronous system. For example, see U.S. Pat. No. 6,137,187 issued Oct. 24, 2000 to Mikhail et al. (hereinafter “the '187 patent”), where the stator of the generator is directly connected to the electric power distribution grid, and the rotor of the generator is connected in turn to the distribution grid through a first AC/DC converter, in turn connected in cascade to a second AC/DC converter whose output is connected to the three-phase power distribution grid. Both the first and second converters are inverters. In this system, the second converter, i.e., the grid side converter also controls the reactive power applied to the grid.
The system described in the '187 patent also includes an electromagnetic torque controller and a blade pitch angle controller. The controller of the electromagnetic torque uses a method for controlling based on the orientation to the stator magnetic field. The blade pitch angle controller carries Out adjustments based on the speed of the generator's rotor, being independent of the electromagnetic torque controller.
However, the doubly-fed asynchronous generator system described in the '187 patent has some disadvantages, resulting from the direct connection of the generator's stator to the three-phase electric power distribution grid. These disadvantages include the fact that disturbances in the grid flow directly to the stator. Thus, significant voltage variations in the grid, such as those due to electrical loads being connected or disconnected, balanced as well as unbalanced short circuits, or similar contingencies, disturb the power production of such generators.
Furthermore, when the doubly-fed asynchronous generators of the prior art are connected to a wind turbine, the system thus formed cannot be connected to the electric power distribution grid when wind speed is low. This is due to the fact that rotor voltage is directly related to the slip, that is, the difference between the rotational speed of the flux and the rotor speed. Since part of the appeal of these systems is that they are able to use relatively low-cost inverters with a relatively small voltage capability, the required low rotational speed of the wind turbine cannot be reached because of the limited voltage ceiling of the rotor side inverter.
Therefore, it would be desirable to develop a generator connectable to a turbine for generating and supplying electric power to an electric power distribution grid that is capable of recovering rotor energy from an asynchronous, or induction, machine, which avoids the problems described above.